A COMPARISON OF 4 METHODS OF ESTIMATING THE RATE AND EXTENT OF CELL-WALL DEGRADATION IN GRASS SILAGES

The feeding value of grass silages, in common with other forages, is c onstrained by the rate as well as the extent of cell-wall degradation in the rumen. Four methods of estimating the rate and extent of cell-w all degradation were compared using six replicates of each of six gras s silages. The purpose was to select one biologically valid method whi ch would be used on a larger number of silages, in order to provide da ta which would be used in the search for a lower cost, rapid method, s uitable for extension work. The four methods compared were: (1) neutra l detergent was used to remove the cell contents from freeze-dried, mi lled silage; the cell wall residue was then incubated in the rumen of sheep in polyester bags (pore size 43 mu m) for different lengths of t ime (0, 3, 8, 16, 24, 45 or 72 h); (2) freeze-dried, milled silage was incubated in the rumen in bags for different lengths of time (as in ( 1)) and the residue treated with neutral detergent; (3) chopped fresh silage (1 cm) was incubated in the rumen in bags for different lengths of time (as in (1)) and the residue treated with neutral detergent af ter freeze-drying and milling; (4) freeze-dried, milled silage was inc ubated in buffered rumen fluid in sealed tubes for different lengths o f time (as in (1)) and then treated with neutral detergent. Methods 2 and 3 had higher standard errors than Methods 1 and 4 and were rejecte d mainly for that reason. Method 4 was selected in preference to Metho d 1 because the cell wall degradation points fell on a smoother curve, a much larger number of samples could be started at the same time wit h the same batch of rumen fluid, the cost was lower and fewer fistulat ed animals were required.